Pharmacokinetics of a new preparation of microencapsulated erythromycin base was studied in 16 healthy subjects. They received 250 mg base 6-hourly or 500 mg 12-hourly for 7 days. The mean maximal serum peaks ( ± SD) after morning doses on days 1, 2, 3, and 7 were 1.4 ± 0.9, 3.2 ± 1.1, 3.6 ± 0.6, 3.5 ± 1.2 mg/l after the 250-mg dose and 3.2 ± 1.5, 3.7 ± 2.1, 3.6 ± 1.8, and 3.0 ± 2.0 mg/1 after the 500-mg dose. The mean 24-hour urine recoveries were 1.8 and 1.2°/o, the serum half-lives were (days 1–7) 1.4–2.1 h and 1.9–2.8 h for the 250-mg and 500-mg doses. The mean areas under the serum concentration curves ( ± SD) were 5.8 ± 2.2, 11.9 ± 2.2, and 15.3 ± 5.1 mg h 1-1 after 250 mg and 14.2 ± 4.9, 16.4 ± 7.6, and 14.3 ± 9.0 mg · h-1 after 500 mg on days 1, 2, and 7. The inter- and intrasubject variability was larger after the 500-mg dose. The pharmacokinetic results indicate that both dosage alternatives are suitable and result in similar steady-state peak levels, but the initia